Metabolism plays an important role in the carcinogenicity of the food-derived heterocyclic amines (HCAs). Metabolic activation occurs by a two-step process involving cytochrome P450-mediated N-hydroxylation and phase II esterification by enyzmes such as N-acetyltransferase (NAT). In humans, two N-acetyltransferases designated NAT1 and NAT2 catalyze N- and O-acetylation of various arylamines, including the O-acetylation of the N-hydroxy-HCAs. In this project, we examined the metabolic activation of HCAs in the human and rat mammary gland by NAT. The mammary gland from rat and human O-acetylated the N-hydroxylamine metabolites of several HCAs including N-hydroxy-IQ and N-hydroxy-PhIP. Mammary gland cytosol from 10 women and lysates from a primary culture of human mammary epithelial cells metabolically activated N-hydroxy-PhIP by NAT-mediated O-acetyltransferase, as measured by the acetyl CoA-enhanced binding of the N-hydroxylatmine to calf thymus DNA in vitro. N-Acetylation of p-aminosalicylic acid, an activity specific to NAT1, but not N-acetylation of sulfamethazine, an activity specific to NAT2, was detected in the mammary gland cytosols and human mammary epithelial cell lysates. Immunohistochemical analysis of human mammary gland sections showed positive staining for NAT1 protein in the epithelial cells lining the mammmary gland ducts. Reverse transcription-PCR analysis showed that mRNA transcripts from both NAT1 and NAT2 were present in human mammary gland; however, no NAT2 catalytic activity was detectable. Our data demonstrate for the first time that human mammary gland is catalytically active toward the metabolic activation of HCA food mutagens, and that this activity is most likely contributed by NAT1 expressed in the ductular epithelial cells of the mammary gland. In light of this study showing that NAT1 is catalytically active and expressed in the human mammary gland, additional studies are needed to determine if the NAT1 genotype is a risk factor for human mammary gland cancer. In order to better assess the role of dietary HCAs in human breast cancer risk, the NAT1 genotype and human mammary gland cancer incidence among women who regularly eat well-done cooked meats is warranted.